demux: cleaner mutex usage

The demuxer layer can start a thread to decouple the rest of the player
from blocking I/O (such as network accesses). But this particular
function does not support running with the thread enabled. The mutex use
within it is only since thread_work() may temporarily unlock the mutex,
and unlocking an unlocked mutex is not allowed. Most of the rest of the
code still does proper locking, even if it's pointless and effectively
single-threaded.

To make this look slightly cleaner, extend the mutex around the rest of
the code (like threaded code would have to do). This is mostly a
cosmetic change.